1. Field of the Invention
The present invention relates to a display device integrated with a touch screen and a method of driving the same, which can enhance touch sensing performance by reducing sensing-signal noise caused by the display driving of an in-cell touch panel.
2. Discussion of the Related Art
Instead of a mouse or a keyboard which is conventionally applied to flat panel display devices, a touch screen (touch sensor) that enables a user to directly input information with a finger or a pen is applied to the flat panel display devices.
A touch screen is applied to monitors such as navigations, industrial terminals, notebook computers, financial automation equipment, and game machines, portable terminals such as portable phones, MP3 players, PDAs, PMPs, PSPs, portable game machines, DMB receivers, and tablet personal computers (PCs), and home appliances such as refrigerators, microwave ovens, and washing machines. Especially, since all users can easily manipulate the touch screen, the application of the touch screen is being expanded.
General touch panels calculate touch coordinates with signals generated by a touch. However, noise caused by the ambient environment of a touch panel makes touch sensing itself difficult, and acts as a cause that makes it difficult to calculate accurate touch coordinates even though a touch is sensed. Since it is practically impossible to perfectly avoid noise, a method for reducing noise occurring in a touch panel is needed.
Recently, in applying the touch screen to display devices, the application of an in-cell type where the touch screen is built in a display panel for slimming is increasing. In such in-cell touch panels, however, noise (caused by display driving) inside a cell as well as noise due to an external cause affect touch driving.
FIG. 1 is a diagram schematically illustrating a related art display device including a touch screen. FIG. 2 is a diagram illustrating an equivalent circuit of a self in-cell touch panel of the related art.
Referring to FIGS. 1 and 2, the related art display device including the touch screen includes a touch panel 10, a display driver 20, and a touch driver 30.
The touch panel 10 includes a touch screen in which a plurality of touch groups are provided, in which case a touch group 12 is provided in units of a certain number of pixels (for example, 64×64 pixels).
A common electrode receiving a common voltage (Vcom) is disposed in each of a plurality of pixels in each touch group 12 of the touch panel 10. In this case, the common electrodes of the respective pixels are used as touch electrodes. In the touch groups 12, the common electrodes are connected to the touch driver 30 through separate touch lines (not shown), and driven as the touch electrodes, thereby sensing a user's touch.
FIGS. 3 and 4 are diagrams for describing a touch sensing method of a touch panel of the related art.
Referring to FIGS. 3 and 4, an in-cell type touch panel has a structure in which a plurality of pixels for display and a touch screen for touch detection are provided together. Due to such a structural characteristic, display and touch sensing are temporally divided and performed.
Specifically, the touch panel determines whether there is a touch and a touched position by using a capacitance differential between adjacent touch groups. A capacitance differential occurs between a touch block touched by a user's finger and an untouched touch block during a touch sensing period (non-display period). The touch driver 30 detects a capacitance differential between adjacent touch blocks (touch groups) 12 to determine whether there is a touch and a touched position.
In detail, touch sensing may be performed in a self touch sensing type. Each of the touch blocks detects an RC delay difference (Δt) between a touched state and an untouched state. When the RC delay difference (Δt) between adjacent touch blocks is equal to or greater than a threshold value, the touch driver 30 determines there to be a touch.
In such a self touch sensing type, a display period and a touch sensing period are divided, and display and touch sensing are performed, in one frame. For this reason, it is unable to sufficiently secure a time for display driving and touch-sensing driving.
FIG. 5 is a diagram showing problems in which touch errors are caused by display driving in the related art display device including the touch screen.
To provide a description with reference to FIG. 5, since liquid crystal has an anisotropic characteristic, the permittivity of the liquid crystal varies depending on a viewing direction, and thus, a capacitance is changed. For example, when a pattern of an image corresponds to a black image, the permittivity of the liquid crystal becomes lower, causing a reduction in a parasitic capacitance. On the other hand, when a pattern of an image corresponds to a white image, the permittivity of the liquid crystal becomes higher, causing an increase in a parasitic capacitance.
Like this, as an image pattern is changed, parasitic capacitances generated in the touch panel are changed, causing ghost-touch noise. Due to the ghost-touch noise, it is unable to accurately detect an actually touched position.
Moreover, an auto-touch error in which a touch is sensed even when there is no actual touch occurs. In detail, as a capacitance change due to noise caused by a screen change becomes greater than a level change of touch raw data, the capacitance change exceeds a touch threshold value, and thus, a touch is sensed even though there is no actual touch.
There is a high probability that the ghost-touch error and the auto-touch error occur when an image pattern of the same touch block is changed from black to white. Furthermore, even when image patterns of adjacent touch blocks differ, the occurrence probabilities of the ghost-touch error and auto-touch error becomes higher.